The ionic mechanisms involved in the generation of several types of postsynaptic potentials have been investigated in the sympathetic ganglia of bullfrogs, by intracellular and sucrose gap recording techniques. The fast EPSP is generated by an increased membrane conductance to both sodium and potassium ions. In contrast to this increased conductance mechanism, the slow postsynaptic potentials, the slow EPSP and the slow IPSP, are generated by different membrane mechanisms. The results indicate that the slow EPSP, in contrast to fast EPSPs, is generated by a decrease of potassium conductance. The slow IPSP on the other hand, appears to be generated by a combination of a decreased sodium conductance and an increased potassium conductance.